During hibernation and spring emergence, black bears suppress their metabolism to 25% and 47% of their basal metabolic rate respectively, but the mechanisms controlling this temperature-independent metabolic suppression are unknown. Studies on hibernating small mammals have suggested that a reduction of mitochondrial electron transport system (ETS) enzymes could play a role in metabolic suppression, yet a basic understanding of black bear mitochondrial enzyme function or mitochondrial abundance in major organs is currently lacking. Citrate synthase enzyme activity in whole tissue is used as a proxy for mitochondrial abundance, whereas additional measurement of citrate synthase activity in isolated mitochondria can elucidate if factors beyond mitochondrial density may contribute to enzyme activity. Our objective was to investigate citrate synthase activity as a proxy for mitochondrial abundance, along with pending further data analysis on protein concentrations of citrate synthase, in homogenized black bear tissues and isolated mitochondria from hibernation, spring emergence, and summer bears. We hypothesized that citrate synthase activity would be significantly lower during hibernation compared to summer in black bear skeletal muscle, kidney, and liver tissue. We found a significant decrease in citrate synthase activity for homogenate kidney samples for hibernation (5.170±1.353 umol per min/g wet wt, n=5, p=0.0004) and spring emergence (7.832±1.611 umol per min/g wet wt, n=9, p=0.0176) compared to summer (in umol per min/g wet wt, 13.62±1.85, n=8). There was a significant decrease in citrate synthase activity for homogenate cardiac samples for hibernation (in umol per min/g wet wt, 45.43±16.16, n=5, p=0.0210) and spring emergence (in umol per min/g wet wt, 45.87±7.72, n=9, p=0.0075) compared to summer (65.72±7, n=9). There was also a significant decrease in citrate synthase activity for isolated cardiac mitochondria in hibernation (394.3±209.3 nmol/min per ug protein, n=5, p=0.0441) compared to summer (633.7±102 nmol/min per ug protein, n=9). There was no significant change in citrate synthase activity for isolated kidney mitochondria in hibernation (33.94±14.78 nmol/min per ug protein, n=5, p=0.1987) compared to summer (65.35±41.55 nmol/min per ug protein, n=9). Liver analysis is pending, all statistics were ANOVA post hoc Dunn’s multiple comparison or Kruskal–Wallis test. In conclusion, while decreased activity of citrate synthase in homogenates could suggest that there may be a decrease in mitochondrial density in kidney and cardiac tissue in hibernation compared to summer, isolated mitochondria results suggest that other factors may be influencing citrate synthase activity in hibernating cardiac tissues and further studies are needed to determine mitochondrial abundance. Research reported in this publication was supported by the UAF Undergraduate Research and Scholarly Activity Program, and an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number P20GM103395. The content is solely the responsibility of the authors and does not necessarily reflect the official views of the NIH. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
menon et al. (Fri,) studied this question.